Water containing suspension solids must be processed to remove substantially all the suspended material before it is discharged to the environment. Equally important costs of water intensive industrial processes can be significantly reduced if suspended solids can be removed from water and the water reused. One cost effective way of removing solids from water is to place the water in a holding tank or pond and allow gravity to act upon the denser solids over time. However, if the suspended solid material has nearly the density of water the process can take too long to be practical. The time considerations can be particularly acute in manufacturing processes which require a continuous flow of great quantities of water, such as papermaking. The manufacture of a single ton of paper may require hundreds of tons of water. Hence, in the modern world, wastewater recycling is critical to manufacturing a cost competitive product.
One widely used technique to remove suspended solids in situations where the solids have a density close to that of water is to inject micro air bubbles into the water containing the dissolved solids. The micro bubbles become attached to the suspended solids and cause them to float to the surface of a container where they may be skimmed off. The bubbles are formed by taking a fraction, typically about 25 percent, of the flow to be clarified, and raising its pressure to about 70 psi. The pressurized water is mixed with about one percent air by volume and the air is dissolved in the water. The water containing the dissolved air is then de-pressurized by passing the water through a nozzle. The air comes out of solution as the water passes through the nozzle, thereby forming air bubbles having a diameter of 20 to 40 microns. The water with the suspended bubbles is then mixed with all the water to be clarified and injected into a rectangular tank which is typically about four to sixteen feet wide, four to ten feet deep, and twelve to thirty feet long.
Bubbles with a diameter of 20 to 40 microns rise through water at a rate of about 16 to 18 inches per minute. As the bubbles rise, they may become attached to solids suspended in the water, causing the solids to float to the top of the tank.
At one end of the tank opposite the water inlet is a sloped surface or beach which extends from beneath the water surface to above the water surface terminating at a sludge containment trough. A scraper on an endless chain drags the floating sludge up the beach and into the sludge containment trough. The sludge as removed from the tank has a solid content of between four and six percent. Clarified water is removed through pipes positioned within the tank at a level about a foot from the tank bottom.
Although a conventional dissolved air clarifier is effective at producing clarified water which can be recycled or safely disposed of, the sludge must be further processed before disposal. The typical process involves further concentrating the solids until the remaining sludge can be cost effectively incinerated or composted. This typically requires additional pressing and dewatering equipment.
What is needed is a dissolved air clarifier which produces sludge with significantly higher solids content.